Method of adding liquid substances to products made by extrusion

ABSTRACT

In products such as fodder pellets made by extrusion of aqueous basic material which at a temperature higher than the boiling point of water is extruded through nozzles of an extruder mouth piece, addition of liquid substances such as for example fat can be carried out by spraying of or immersion into the substances concerned in such a way that the decreasing pressure in the steam-filled cavity of the material during cooling outside the extruder is utilized to overcome the flow resistance of the liquid additives during their inward travel from the surface of the basic material.

The present invention relates to a method for addition of liquidsubstances to products such as fodder pellets made by extrusion whereaqueous basic material at a temperature above the boiling point of wateris extruded through the nozzles of an extruder mouth piece.

For the extrusion of such products as for example fodder and foodstuffsthe basic material is fed to a cylindrical housing and passing throughit towards a mouth piece with one or more nozzles by one or two wormsrotating in the housing. Having passed through the nozzles the extrudedmaterial can be cut by knives into pieces, pellets, of a suitablelength.

In an extruder with two worms these are designed so that in combinationwith the housing they constitute a pump with positive displacement. Suchan extruder can, therefore, work at a high pressure and independently ofthe interior friction of the material so that fat and correspondingmaterials can be added to the basic material prior to extrusion.However, the double worms have to be made and installed with greatprecision, and this type of extruder is, therefore, relativelyexpensive.

An extruder with only one worm is simple and correspondingly cheap, butworks at a relatively low pressure which is determined especially by theviscosity and structure of the basic material. When the viscosity isreduced, a tendency to internal return flow causes a limitation of theamount of additives, especially fat, which can be added to the basicmaterial prior to extrusion.

To obtain a higher fatty content in the finished pellets than isnormally possible by means of a single-worm extruder, a certain amountcan be added to the finish-extruded pellets through the surface, eitherby spraying or in a rotary coater drum.

It is thus not possible to obtain a homogenous product in this way assaturation of surface pores impedes transport of liquid to the corematerial. The inward travel of the liquid effected by capillary forcesas motive power will be opposed by a growing pressure from the air to beousted through the fat-saturated outer layer. In many cases it will,therefore, be impossible to make products with a desired fatty content.

These drawbacks are avoided according to the present invention by theaddition of the liquid substances being effected by spraying of orimmersion into the substances concerned in such a way that thedecreasing pressure in the steam-filled cavity of the material duringcooling outside the extruder is utilized to overcome the flow resistanceof the liquid additives during their inward travel from the surface ofthe basic material.

With this procedure the addition, therefore, takes place underconditions which do not involve the need for replacement of air as withthe method used so far, where on the assumption that the material wouldotherwise become loose and crumbling the extruded pellets have beenallowed to cool considerably before the addition. With the methodaccording to the invention where addition is, therefore, madeimmediately after the extrusion, a pressure gradient will, on the otherhand, supplement the capillary effect so that a rate of absorption canbe obtained which corresponds to what can be obtained with a double-wormextruder.

During passage of the material through the extruder nozzles the pressuredrops, thus converting the water content in the material abruptly fromliquid to steam phase. This results in an expansion caused by theevaporation of the water particles whereby a lot of steam-filled poresare created. During this cooling which is initiated immediately afterthe material has left the mouth piece, the steam will re-condense. Avacuum is thus created in the material which will draw simultaneouslyadded additives towards the core of the pellets.

Addition of substances--fat or other liquids--can, of course, be made inthe manner described irrespective of the type of extruder used when onlythe assumed pressure and temperature sequences make it possible.

What is claimed is:
 1. A method of adding an additive substance inliquid form to products comprising extruded foodstuffs and fodderpellets, which method comprises:extruding an expandable aqueous basicfoodstuff material, at a temperature higher than the boiling point ofwater and at a pressure sufficient to maintain the water in saidexpandable aqueous foodstuff material in its liquid phase, through anozzle of an extruder mouthpiece to form an extrudate, whereby inpassing through and out of said nozzle the pressure in said aqueousbasic product material extrudate is rapidly lowered, and thus rapidlyconverting the water in said aqueous basic material into its steam phaseresulting in a simultaneous expansion of said extrudate caused byevaporation of said water therein; immediately after extrusion of saidaqueous basic product material extrudate from said nozzle and thesimultaneous expansion of said extrudate, spraying an additive substancein liquid form onto the surface of said extrudate for coating saidsurface therewith; and immediately after coating said extrudate withsaid additive substance in liquid form, cooling said coated extrudate topromote absorption of said coating inwardly into said extrudate,whereby, during cooling of said coated extrudate, condensation of steamtrapped therein creates a vacuum therewithin drawing said coated liquidadditive thereinto towards the core thereof under a pressure gradientbetween said vacuum and the ambient temperature.
 2. A method of addingan additive substance in liquid form to products comprising extrudedfoodstuffs and fodder pellets, which method comprises:extruding anexpandable aqueous basic foodstuff material, at a temperature higherthan the boiling point of water and at a pressure sufficient to maintainthe water in said expandable aqueous foodstuff material in its liquidphase, through a nozzle of an extruder mouthpiece to form an extrudate,whereby in passing through and out of said nozzle the pressure in saidaqueous basic product material extrudate is rapidly lowered, and thusrapidly converting the water in said aqueous basic material into itssteam phase resulting in a simultaneous expansion of said extrudatecaused by evaporation of said water therein; immediately after extrusionof said aqueous basic product material extrudate from said nozzle andthe simultaneous expansion of said extrudate, immersing said extrudatein an additive substance in liquid form for coating the surface of saidextrudate with said additive substance in liquid form; and immediatelyafter coating said extrudate with said additive substance in liquidform, cooling said coated extrudate to promote absorption of saidcoating inwardly into said extrudate, whereby, during cooling of saidcoated extrudate, condensation of steam trapped therein creates a vacuumtherewithin drawing said coated liquid additive thereinto towards thecore thereof under a pressure gradient between said vacuum and theambient temperature.